feat: comprehensive project improvements

- Fix 14 Clippy warnings across server and bot-sdk
- Add 67 unit tests (32 bot-sdk, 34 server, 1 doctest)
- Add Prometheus metrics endpoint (/api/metrics)
- Add structured JSON logging (EIFELDC_LOG_FORMAT=json)
- Add release workflow (Docker push + GitHub Release + Tauri builds)
- Add rate limiting middleware (EIFELDC_RATE_LIMIT)
- Add CORS restriction (EIFELDC_CORS_ORIGINS)
- Add session token expiry (EIFELDC_SESSION_TTL)
- Add input validation (username/password/homeserver length limits)
- Add upload size limit (EIFELDC_MAX_UPLOAD_MB)
- Upgrade Tauri client from v1 to v2
- Add session store with SQLite persistence
- Add proper error types and cleanup across all crates
- Format all code with cargo fmt
- Update CI pipeline with fmt, clippy, test, frontend, and Tauri checks
- Add README with full API reference and setup guide

Makefile

@@ -0,0 +1,49 @@
+.PHONY: build check test dev clean docker docker-up docker-down docker-logs docker-build health backup setup
+
+build-server:
+	cargo build --release -p eifeldc-server
+
+build-bot-sdk:
+	cargo build --release -p eifeldc-bot-sdk
+
+build-frontend:
+	cd client/src-ui && npm ci && npm run build
+
+build: build-server build-frontend
+
+check:
+	cargo check -p eifeldc-server -p eifeldc-bot-sdk
+	cd client/src-ui && npm ci && npm run build
+
+test:
+	cargo test -p eifeldc-server -p eifeldc-bot-sdk
+
+clean:
+	cargo clean
+	rm -rf client/src-ui/node_modules client/src-ui/dist
+
+dev:
+	cargo run -p eifeldc-server
+
+docker-build:
+	docker build -t eifeldc:latest .
+
+docker-up:
+	cd infra && docker compose up -d
+
+docker-down:
+	cd infra && docker compose down
+
+docker-logs:
+	cd infra && docker compose logs -f eifeldc
+
+docker-restart: docker-down docker-up
+
+health:
+	bash infra/scripts/healthcheck.sh
+
+backup:
+	bash infra/scripts/backup.sh
+
+setup:
+	bash infra/scripts/setup.sh